Such a data acquisition system requires detecting and measuring of transmitted X-rays over a wide range of X-ray intensities corresponding to a wide range of beam attenuations. This range is defined between the maximum intensity of the unattenuated beam at maximum aperture at the high end of the range, and by the full attenuation of a very dense object at minimum aperture at the low end of the range.
Existing methods and apparatus for detecting and measuring radiation employ a scintillator driving a photomultiplier. The scintillator, in response to impinging photons, generates optical energy (or light). The photomultiplier generates electrical current in response to detecting optical energy (or light). The output of the photomultiplier is usually coupled to analyzing circuits which operate either in the charge integration mode or the single photon counting mode. The output from the photomultiplier is theoretically a series of pulses. The frequency of the occurrence of these pulses is directly related to the frequency of radiation emission events or photons which are detected. The magnitude of each such pulse is related to the energy of the photons which are detected. Hence, there are two qualities of the output signal from the photomultiplier which may be used to develop a signal suitable for giving an output indication. When charge integration is used the electronic current generated by one of the photomultiplier electrodes is integrated and measured. The single photon counting mode on the other hand consists of detecting and counting discrete electronic pulses generated by one of the photomultiplier electrodes. Neither technique is sufficient to measure a wide range of X-ray intensities. The charge integration technique is incapable of measuring very low levels of X-ray radiation because of the presence of fluctuating dark current of the photomultiplier. The single photon counting technique is incapable of measuring very high levels of X-ray radiation because of the slow response time of typical scintillation crystals which cause individual X-ray pulses to blend together so they can no longer be resolved into individual pulses. A so-called pile up effect occurs whenever a new pulse is detected before the previous pulse has become extinguished. The incidence of this effect is related to the count rate. Hence the probability of its occurrence is all the greater the higher the pulse rate.
It is an object of the present invention to provide a method and apparatus which overcomes the deficiencies of the previously described prior art. Accordingly the present invention allows detection of X-ray data over a wide range of X-ray intensities by using the analyzing circuits simultaneously operating in charge integration and in single photon counting modes.
It is also another object of the present invention to provide a method and apparatus in which a significant power and cost savings can be achieved. Economic savings occur due to the fact that high-level charge integration signal in the system is generated by one of the low gain electrodes of the photomultiplier and therefore does not dissipate as much power in the bias circuitry. This power advantage is especially significant when a large array of detectors is used in a multichannel system or when the detectors are intended for use in the space environment where power is at a premium.